Animal nervous systems are not ‘new’


  • Early forms of ‘sense-react’ existed in prokaryotes (not animals)


  • During the Cambrian explosion, specialized nervous systems appeared
    • allowed animals to sense environment
    • allowed animals to respond quickly


  • Cnidarians are the simplest organisms with nervous systems
    • nerve nets scattered throughout body
    • control contraction & expansion of gastrovascular cavity

Nerve net in jellyfish

Nervous systems consist of circuits of neurons and cells

  • In complex animals, axons of many neurons are bundled into ‘nerves’ with other tissues


  • Fibrous-like nerves channel information flow along targeted routes in the nervous system


  • Nerve circuits differ across the animal phylogeny
    • sea stars = radial nerves connected to central ring
    • bilaterial animals - cephalization

Central nervous systems (CNS)

  • Many animals have neurons that carry out integration (i.e., perception) formed into a CNS


  • Planaria (flat worms) represent the first defined CNS
    • small brain and 2 nerve cords
    • only a few hundred neurons



  • Higher animals have more complicated brains and nerve cords with ganglia
    • ganglia - clusters of nerves at at key points in the body

Nervous system organization also matches lifestyle

Peripheral nervous system (PNS) relays info to CNS

Vertebrate nervous system

  • CNS develops from hollow dorsal nerve cord
    • central canal of spinal cord
    • hollow ventricles of brain


  • Brain & spinal cord have grey and white matter
    • grey = neuron cell bodies, dendrites, terminals
    • white = bundled axons


  • White matter exists on the interior of brain where neuron signalling is turned into function
    • learning, emotions, processing, etc.


  • Grey matter exists on the outer layer of the spinal cord to link CNS with PNS

Spinal cord relays info to and from brain

Can act independently for simple rapid reflexes

PNS plays a large role in animal movement/homeostasis


  • Sensory information reaches the CNS along PNS neurons
    • afferent neurons = to carry towards


  • CNS processes information and instructions travels to muscles, glands, etc.
    • efferent neurons = to carry away


  • Nerve bundles include both afferent and efferent neurons

PNS plays a large role in animal movement/homeostasis


  • PNS has 2 efferent components
    • motor system
    • autonomic nervous system


  • Motor system carry signals to skeletal muscles
    • voluntary (raise a hand)
    • involuntary knee-jerk


  • Autonomic system regulates involuntary smooth (organs) and cardiac muscles
    • Sympathetic, Parasympathetic, Enteric
    • Enteric = digestion, pancreas, gallbladder

Sympathetic and Parasympathetic are antagonistic

Calming and self-maintenance (para) vs Arousal and energy generation (symp)

Vertebrate brain is specialized by region


Forebrain =


Midbrain =


Hindbrain =


Across vertebrates, sizes of each region reflect importance of functions to each animal

Birds and mammals have larger brain:body size ratios

Human brain


100 billion neurons make 100 trillion connections


Cerebrum (enhanced region of forebrain) is essential for language, cognition, memory, consciousness and awareness

Human brain


100 billion neurons make 100 trillion connections


Cerebrum is essential for language, cognition, memory, consciousness and awareness


  • Cerebrum is also regionized ( Don’t Memorize)
    • Frontal lobe = temperament and decisions
    • Temporal lobe = hearing
    • Paretial lobe = touch and integration
    • Occipital lobe = vision